Scanning electron microscopes include a chuck for supporting an inspected object, a mechanical stage for moving the chuck, interferometer mirrors, and an interferometer.
The interferometer mirrors are mechanically coupled to the mechanical stage by a metal coupling element.
The interferometer is configured to illuminate the interferometer mirrors with a laser beam in order to determine the location of the chuck.
The mechanical coupling between the interferometers mirrors and the mechanical stage introduced unpredicted relative movements between the interferometer mirrors and the chuck.
The movement of the mechanical stage heats the mechanical stage, the chuck, the metal coupling element and the interferometer mirrors.
Differences between the thermal expansion coefficients of the chuck, the interferometer mirrors and the metal coupling element cause the interferometers mirrors to be deformed and/or to move in relation to the chuck in an unpredicted manner.
The unpredicted movements and the deformation of the interferometer mirrors introduce an uncertainty in the determination of the location of the chuck.
There is a growing need to provide a device for reducing the uncertainty in the determination of the location of the chuck.